The present invention relates to a gear housing of a driving device equipped with a motor.
Japanese Laid-Open Patent Publication No. 2001-103705 discloses a driving device used in a vehicle wiper device or the like. The driving device includes a motor, which has a rotary shaft, and a reduction section, which reduces rotation of the rotary shaft before outputting the rotational force. The reduction section includes a gear housing, which is secured to a motor, a reduction gear, which is accommodated in the gear housing, and an output shaft, which projects outside the gear housing. The reduction gear includes a worm, which is secured to the rotary shaft, and a worm wheel, which meshes with the worm. The output shaft is coupled to the worm wheel. The gear housing has a support cylinder, which supports the output shaft to prevent the axis of the output shaft from tilting. The gear housing also has reinforcing ribs, which radially extend from the outer circumference of the support cylinder. The reinforcing ribs improve the strength of the support cylinder and prevent the output shaft from being tilted by a load applied from the outside.
The gear housing is formed by casting. During casting, the gear housing contracts as it is solidified and cooled. At this time, the reinforcing ribs, which extend radially, cause a relatively large tension to act on the support cylinder along the radial direction. This decreases the circularity of the inner circumference of the support cylinder by a large amount. When the circularity of the inner circumference of the support cylinder is low, the axis of a sliding bearing, which is secured to the inner circumference of the support cylinder, may be tilted by an intolerable degree or lubricant oil between the sliding bearing and the output shaft may leak. Therefore, the support cylinder need to go through a cutting process or a sizing process after casting the gear housing in order to increase the circularity of the inner circumference of the support cylinder.